Controlled release fertilizer particles which have remarkably high resistance to attrition and a method for their preparation are disclosed in Moore, U.S. Pat. Nos. 4,711,659 and 4,804,403. According to those patents, controlled release fertilizer particles are obtained by reacting a water-soluble central mass of plant food compound containing reactive functional groups such as urea, in particulate form, with a chemical coupling agent followed by reaction with a coating material, such as a polyol, to provide a water-insoluble polymer coating or sealing layer on the plant nutrient. The plant nutrient and sealing layer are chemically bonded to each other through the coupling agent. Specifically, the coupling agent reacts with and connects itself to functional group on a water-soluble central mass of plant nutrient to form generally a base coating having additional reactive groups. A water-insoluble coating or sealing layer then is bonded to the base coating through its reaction with the additional reactive groups on the base coat. Thereafter, multiple reacted layers of alternate applications of coupling agent and sealing layer are formed as a coating to a desired thickness. The coated fertilizer particles are highly resistant to attrition even under extreme vibration, impact and abrasion.
Although the coupling agent as described in the '403 patent can be any material which is reactive with the functional groups of the plant nutrient and with functional groups making up the water-insoluble coating, the polyisocyanates such as diphenylmethane diisocyanate, are preferred. Further, although the water-insoluble coating material can be any of various coating materials as described in the '403 patent, preferably the coating material is a polyol. The disclosure of the aforesaid '659 and '403 patents exclusively licensed to the assignee of the present application are incorporated herein by reference.
Although the aforesaid '659 and '403 patents provide highly advantageous slow release fertilizers, an improved system is desired to provide uniform coatings in the large quantities of fertilizers needed for commercial application.